The present invention relates to a method of thermal treatment of a product web by blowing of steam, as well as to a corresponding device for thermal treatment.
During a process of thermal treatment of a continuous product web the steam is blown in a recirculating air process through nozzle boxes onto the product web and after heating is supplied again. A device for performing this process which is identified herein below as a steam dryer, has a steam-tight housing with an inlet and an outlet slot and a transporting device for the product web. Frequently several treatment fields are arranged one behind the other, and each field is provided with a device for supplying of steam in the recirculating air process.
The thermal treatment can include a drying, a drying with a fixing, or a pure fixing. All fields of the steam dryer or only some of the fields can be operated with steam. The continuous product webs to be treated are preferably textile webs. Also they can be represented by paper webs or foil webs or the like. The product webs are transported by a transporting device, for example with tension chains, with a roll table or with a sieve band through the steam dryer. The transporting device can also have a nozzle box provided with air nozzles for free-floating guidance of the product web.
The advantages of a thermal treatment, in particular a treatment with pure hot steam, which provides especially high efficiency or quality improvement are described already in the book "Dryers and Drying Processes" of K. Kroll, Springerverlag 1959. In order to use these advantages in practice, there are some difficulties to build the hot steam dryer so tight that no air can penetrate into it. Critical places at which leak stream can occur are the inlet and outlet slots as well as all other openings, for example for conductors, shafts or doors.
In a method and device disclosed in the German patent document DE 195 46 344 the penetration of air is substantially suppressed by providing all critical places in the bottom of the housing and arranging suction boxes which extend in the housing at both sides of the inlet slot and the outlet slot. The steam produces in the whole inner space of the housing an overpressure which is especially high in the vicinity of the bottom. Also, in the regions which are in communication at the suction side with the blowers a small overpressure of the atmosphere is produced. Excessive steam escapes through the inlet slot and the outlet slot and is aspirated by the suction boxes.
The problem is however that the product webs transported through the inlet slot is substantially colder than the steam in the inner space of the housing. Thereby this can lead to an undesired condensation of the steam in the inlet region.
The German document DE 27 27 971 discloses a device for thermal treatment of a continuously moving product web, in which for reducing the entry of space air in a treatment chamber at the outer side a nozzle system is provided as a sealing in the product web inlet and outlet openings provided in a wall of the treatment chamber. A nozzle system contains a first nozzle pair which adjoins the chamber opening and is connected with a point of the treatment chamber located at a negative pressure, as well as a following second nozzle pair which is connected with the pressure side of a fan communicating at a suction side with the atmosphere.
The arrangement of the nozzle system which is known from German document DE 27 27 971 in a steam dryer is however not possible since steam dryers operate with overpressure. In particular, in the places in the interior of a steam dryer with under pressure, the danger of leakage streams is increased. By the connection of a first nozzle pair with a place located at a negative pressure in the steam dryer, the air which is aspirated by the fan from the atmosphere and supplied to the second nozzle pair will go into the steam dryer. A penetration of space air can therefore not be prevented reliably. Also it is disadvantageous that the cold web which is transported through the product web inlet opening can cause in many instances an undesired condensation of the steam.
A further drying device, in particular for paper webs is disclosed in German document DE-A 42 26 107. This drying device has a dryer with additional nozzle boxes at both sides of the inlet and outlet slot. Conduits for pre-heated treatment air lead to the nozzle boxes. The nozzle boxes are provided with nozzle openings whose blowing direction is oriented at an acute angle to the web guiding plane on the inlet slot. For this purpose the nozzle boxes in section are profiled in a saw-toothed manner. The penetration of cold surrounding air through the inlet slot must be suppressed by a passing adjustment between the outlet speed of the blowing jets and the negative pressure produced in the interior of the dryer. The negative pressure prevents escape of solvents which become volatile during drying. Contrary to this, a steam dryer must be operated in the over pressure. Therefore the method disclosed in the German document DE-A 42 26 107, which makes possible a pre-heating of the product web and a suppression of the penetration of cold surrounding air can be used in the steam dryer. Disadvantage is in particular the fact that in each case the pre warmed treatment air penetrates into the dryer. In a steam dryer no air must however penetrate into it.
A thermal treatment device which is different from the devices for thermal treatment of continuous webs, namely a suspension loop steamer in which the product web is guided through the treatment on supporting bars of the rotating chain in a freely suspending loop is disclosed in the German document DE 29 51 299. This suspension loop steamer is provided with an air or steam sluice on an inlet slot located in the bottom. It does not disclose any information on the construction and operation of the sluice. In contrast to a steam dryer, in which steam is blown on a continuously moving product web and is continuously circulated, the steam flow and the product web speed in a suspension loop steamer is substantially smaller, or in other words the requirements to a sluice are lower.